A microstructural indicator of shear sense in volcanic rocks and its relationship to porphyroblast rotation in metamorphic rocks

An asymmetric arrangement of microfolds in flow layering adjacent to phenocrysts indicates the sense of shear and is a potential indicator of the local flow direction in flow-layered volcanic rocks. This microstructure is analogous to a similar arrangement of microfolds around rotated porphyroclasts in some metamorphic rocks. It is also relevant to the incorporation of inclusion trails in porphyroblasts that grow while undergoing rotation relative to the matrix foliation, suggesting that asymmetrical microfolds may form outside porphyroblasts during their growth and relative rotation. Asymmetrical microfolds adjacent to garnet porphyroblasts have been observed in some metamorphic rocks.-Author